Azolla is useful as biofertilizer in rice fields mainly by virtue of its

Options

A. Symbiosis with Nostoc
B. Ability to live in water
C. Symbiosis with Anabaena azollae
D. Akinetes and Heterocysts are found

Answer

**Step-by-Step Solution:**

1. **Understanding Azolla**: Azolla is a floating aquatic fern that is commonly used as a green manure in rice fields. It thrives in water-logged conditions, which makes it suitable for rice cultivation.

2. **Role of Azolla in Rice Fields**: Azolla is beneficial in rice fields primarily due to its ability to enhance crop growth and improve soil fertility. It does this by fixing atmospheric nitrogen, which is essential for plant growth.

3. **Symbiotic Relationship**: The nitrogen-fixing ability of Azolla is attributed to its symbiotic relationship with a specific cyanobacterium known as Anabaena azollae. This relationship allows Azolla to utilize atmospheric nitrogen, converting it into a form that plants can absorb.

4. **Key Structures**: Anabaena azollae contains specialized structures called heterocysts and akinetes. Heterocysts are responsible for nitrogen fixation, while akinetes serve as a means of survival under unfavorable conditions.

5. **Evaluating Options**:
- **Option 1: Symbiosis with Nostoc**: While Nostoc is another cyanobacterium, it is not the one associated with Azolla. Thus, this option is incorrect.
- **Option 2: Ability to live in water**: While Azolla does live in water, this characteristic alone does not explain its role as a biofertilizer.
- **Option 3: Symbiosis with Anabaena azollae**: This is the correct answer, as this symbiotic relationship is what allows Azolla to fix nitrogen and enhance soil fertility.
- **Option 4: Akinetes and heterocysts are found**: This statement is true regarding Anabaena, not Azolla itself, making this option incorrect.

6. **Conclusion**: The correct answer is that Azolla is useful as a biofertilizer in rice fields mainly due to its symbiosis with Anabaena azollae, which enables nitrogen fixation.

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